Basic arrow definitions, based on
Generalising Monads to Arrows, by John Hughes,
Science of Computer Programming 37, pp67-111, May 2000.
plus a couple of definitions (returnA and loop) from
A New Notation for Arrows, by Ross Paterson, in ICFP 2001,
Firenze, Italy, pp229-240.
See these papers for the equations these combinators are expected to
satisfy. These papers and more information on arrows can be found at
http://www.haskell.org/arrows/.

Conditionals

Choice, for arrows that support it. This class underlies the
if and case constructs in arrow notation.
Any instance must define left. The other combinators have sensible
default definitions, which may be overridden for efficiency.